This invention relates to antennas, and more particularly to helical-type antennas and arrays thereof.
Parabolic reflector antennas are widely used for INMARSAT A and B.
The specifications of INMARSAT A and B require the frequency range between 1.525 and 1.6465 GHz, 20 dBi or higher gain, and the axial ratio of 2 dB or less. But the caliber of parabolic reflector antenna is as large as about 85-90 cm, and its pedestal (plinth) is also heavy to satisfy these requirements.
Furthermore, an antenna efficiency of a parabolic reflector antenna is generally about 70%, and it""s been desired to develop the efficient antenna which can be miniaturized.
Then, a helical antenna has been developed as shown in the following patent and patent applications: (1) Japanese Patent Publication No. 8-2005, (2) Provisional Publication No. 7-235829, and (3) Provisional Publication No. 5-259734. These antennas are made to supply electric power through a waveguide by arranging helix conductors which are longer than a wavelength.
With these helical antennas, it""s possible to obtain a circularly polarized wave with a good axial ratio as radiation wave to the direction of an axis of a helix by defining a pitch angle and a length of a helix. Moreoever, the gain is increased and simultaneously, the axial ratio is also improved by increasing the number of helix turns and with a longer helix length.
However, the size of a helix in the axis direction becomes long and the size of antenna becomes large as the whole, if the number of turns of helix conductor of each element antenna is to increase, for improvement in a gain and an axial ratio. Therefore, the number of turns of helix is restricted naturally.
For example, the theoretical gain of the helical antenna with 1.5 turns is 7-8 dBi and an axial ratio will be 3 dB or more
On the other hand, the input impedance of helix is as much as or more than 100 ohms, and a matching circuit is required in order to connect with the power feeder of 50-ohm system.
For example, in the helical antenna as shown in FIG. 8, the dielectric cylinder 1 with a helix conductor 2 is arranged on the conductive plate 3, the matching circuit 4 is formed on the conductive plate 3, and electric power is supplied through this matching circuit 4.
This matching circuit consists of a xcex3g/4 adjustment circuit which is constituted from a micro strip line, and an adjustment circuit using the spatial combination with a metal plate and the ground.
However, with the structure which establishes such an adjustment circuit in the exterior of an element antenna, it is difficult to adjust the power feeding phase to each element antenna, when a helical antenna array is constituted.
In short, the pattern of the whole power feeding circuit including the matching circuit becomes complicated, since it is necessary to change the position of a matching circuit according to the position of an element antenna, even though element antennas are arranged so that the power feeding phases are the same.
Consequently, it is also difficult to miniaturize the whole helical antenna array.
Moreover, in the helical antenna array shown in the above-mentioned official report (1), (2), and (3), there are problems that:
it is hard to fix a probe certainly;
it is not possible to measure the characteristic by a probe;
it is hard to set the impedance matching with helix conductor; since it is made to insert the joint part (probe) of helix conductor (coil) into waveguide from the front of an conductive plate.
Moreover, it is required to use the thick pair of upper and lower conductive plates constituting a waveguide in order to acquire the stable characteristic with the structure of power feeding through waveguide.
Then, there occurs a problem that the weight increases and its pedestal also becomes heavy for the reason, as shown in each above-mentioned official report.
Furthermore, it is impossible to raise a gain without increasing the size and weight of the whole antenna, since it is required to increase the number of element antennas and the area of an conductive plate in order to obtain a predetermined gain with the helical antenna array shown in the above-mentioned official reports.
The object of this invention is to constitute a helical antenna and a helical antenna array which is small and light with a height gain.